Why dotMobi?
The core mission of dotMobi is two-fold, to become the most ubiquitous domain name for the mobile web (i.e. get the consumer to think .mobi rather than .com when using the Internet from their handset) and to become the critical link between mobile operators, handset manufacturers, content providers / brands and users to make phones the tool of choice for the mobile web.
A mobile device requires an entirely different user experience than that of a traditional PC or laptop. Just because a traditional website is now viewable on many phones, that does not equate to a tangible, fulfilling user experience. Unfortunately, that is exactly what many companies provide today. Obviously, the device is smaller, the screen space is limited, and the traditional QWERTY keyboard input method is not ideal. Further, the web is probably being accessed on a mobile phone while the end-user is driving, on the move, or otherwise engaged by some other activity or media device. If you currently have a transactional web site, then I strongly urge you to get a dotMobi version as well. If you don't, you will be losing business.
There's a little more to it than just getting a .mobi domain. There's still the effort required to reformat your site to work well on small displays. It's simple enough to maintain a "mobile friendly" site once you make the effort to support it. There are some desktop applications that will convert your exsisting site to a mobile formatted site. But I would only recommend that for static sites that don't change very often. On my own personal site, which has a lot of dynamic content, I chose to just detect if the browser requesting the page is a mobile one and reformat on the fly. There are lots of resources to help a web developer navigate the pitfalls of mobile browsers.